Separate investigations have found that the lack of oxygen was not the primary cause of any of the deaths.

But procedures have been tightened following the cases in 2017 and 2018.

Now medical director John Oxtoby has issued a statement following the deaths.

Royal Stoke University Hospital

He said: “We are aware of three instances in 2017 and 2018 where portable oxygen cylinders had run out on the trolleys of patients who later died.

“Following investigation, the running out of oxygen was not found to be the primary cause of death in any of these cases.

“However, as part of the learning from these instances, we have had a progressive tightening of controls which have included the development and strengthening of standard operating procedures to monitor those patients using this equipment. We have also purchased oxygen cylinders with alarms on as an additional way of alerting staff when levels are getting low.

“We have, of course, apologised to the families of those affected.”

The region's clinical commissioning groups (CCGs) are aware of the incidents. It is not known whether inquests have been held into the deaths of the three patients and the hospital has not disclosed when the deaths occured.

CCG official Doctor Steve Fawcett said: “We are aware of the incidents at Royal Stoke University Hospital's emergency department in 2017 and 2018. These three patients were very ill when they came to the hospital and there is no evidence to show that their deaths were the direct result of oxygen cylinder problems.